Interpretive Summary: Iris yellow spot virus and its onion thrips vector are important pests of onion, the latter often managed intensively with insecticide applications. In this project we investigated the relationship of iris yellow spot to a number of factors, including thrips populations, soil properties, and grower management practices in Colorado onion fields. We found that the number of thrips per plant and the percentage of plants with iris yellow spot were associated with several soil factors. The incidence of disease also was related to how densely onion plants were planted and the number of thrips. These results could help to develop a risk index for iris yellow spot to predict potential risk from iris yellow spot and its thrips vector.

Technical Abstract:
Iris yellow spot virus and its onion thrips vector (Thrips tabaci) are yield limiting pests of onion in the western U.S. This 2-year project investigated the relationship of iris yellow spot to thrips populations, soil properties, and grower management practices in Colorado onion fields. A random systematic sampling strategy was utilized to assess the association of the incidence of iris yellow spot, thrips density, the number of onion plants, and yield from plots placed on a 0.2 ha sampling grid in six onion fields during 2005 and 2006. Soil cores from these plots (218 total) were collected and analyzed for nutrient levels, salt content, pH, and organic matter for correlation analyses. Thrips density was negatively associated with boron, calcium, and cation exchange capacity, and positively associated with iron, pH, and sodium. The incidence 1 of iris yellow spot at harvest had similar associations with these factors, and also was positively associated with levels of copper, hydrogen, potassium, and zinc. Disease incidence was negatively associated with organic matter content. Thrips-days, a metric of season-long thrips density, had a clear non-linear association with the incidence of iris yellow spot at harvest. A negative association between plant population and the incidence of iris yellow spot also was detected in fields where the disease incidence was relatively high. Results could contribute to the development of a risk index for iris yellow spot to predict potential risk from iris yellow spot and its thrips vector.